The present invention relates generally to remote reporting systems for elements in digital transmission line systems and, more particularly, to a digital line element such as a repeater or network interface unit (NIU) that may remotely report its status (such as its operating mode or location) to a remote facility interconnected to a digital transmission line. In this way, for example, the invention may assist a telephone company technician in identifying, from a test location, the location of a malfunctioning line element among a string of line elements such as multiple repeaters and an NIU.
The present invention may be used with digital transmission lines generally, including, for example, the Bell Telephone System in the United States. The data, or xe2x80x9cpayloadxe2x80x9d, signals on such transmission lines are typically sent differentially on a Tip-Ring pair. Payload signals are received by the telephone company central office, and generally, are transmitted, via cables, to a series of regenerative signal repeaters and an NIU in a T1 span line. Such repeaters are spaced along the cables approximately every 6,000 feet. The first repeater receives the data from the central office, but, because of transmission line losses, noise, interference, and distortion, the signal will have degenerated. The repeater recognizes the presence or absence of a pulse at a particular point in time and thereafter, if appropriate, regenerates, or xe2x80x9cbuilds up,xe2x80x9d a clean, new pulse. The first line repeater (or xe2x80x9csignal repeaterxe2x80x9d or xe2x80x9cregenerative repeaterxe2x80x9d) sends the regenerated, or repeated, signal to the next line repeater, stationed approximately one mile away. The last repeater then transmits its pulses to an NIU at the remote end of the span line typically located at the point of demarcation between the network and the customer premises.
The Bell Telephone System has widely utilized time multiplexed pulse code modulation systems. Such systems have generally been designated as xe2x80x9cT carriers.xe2x80x9d The first generation of multiplexers designed to feed the T1 system was the D1 channel bank. Channel banks have evolved through the D5 series. The xe2x80x9cDxe2x80x9d channel bank provides multiple DS-1 signals that are carried on the T1 systems. Each T1 system carries twenty-four two-way channels on two pairs of exchange grade cables. One pair of cables provides communication in each direction.
The data to be transmitted over the cables, such as speech, may be sampled at a rate of 8,000 hertz, and the amplitude of each signal is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into a binary form. Representative binary pulses appear on the transmission lines.
The binary form of each sample pulse consists of a combination of seven pulses, or bits. An eighth bit is added to the end of the combination, or byte, to allow for signaling.
Each of the twenty-four channels on the T1 system is sampled within a 125 microsecond period (equivalent to {fraction (1/8,000)} of a second). The period is called a xe2x80x9cframe.xe2x80x9d Within each frame, an additional, synchronizing bit is added in order to signal the end of a frame. Otherwise, a single error might cause future representations of the data on the transmission line to be misunderstood by the receiving apparatus.
Since there are eight bits per channel and there are twenty-four channels, and there is one pulse at the end of each frame, the total number of xe2x80x9cbitsxe2x80x9d needed per frame is 193. Thus, the resulting line bit rate for T1 systems is 1.544 million bits per second.
A coding system is typically used to convert the analog signal to a digital signal. The system guarantees some desired properties of the signal, regardless of the pattern to be transmitted. The most prevalent code in the United States is bipolar coding with an all zero limitation (also called xe2x80x9cAMIxe2x80x9d for Alternative Mark Inversion).
With bipolar coding, alternate xe2x80x9conesxe2x80x9d are transmitted as alternating positive and negative pulses, assuring a direct current balance and avoiding base-line wander. Further, an average density of one pulse in eight slots, with a maximum of fifteen zeros between xe2x80x9cones,xe2x80x9d is required. This is readily obtained in voice-band coding, however, by simply not utilizing an all-zero word.
Another arrangement, also used to guarantee density with a bipolar code, replaces strings of zeros with two successive pulses of the same polarity, allowing its identification and removal at the receiving end. This arrangement, called B8ZS (for Bipolar with 8-Zero Substitution) is also in considerable use. Other coding arrangements (such as B3Zs and 4B3T) have also been established.
Signals that violate the rules established in a particular system are detected as types of errors. Thus, for example, under a bipolar coding scheme, two positive pulses should never occur in sequence (except in B8ZS encoded all-zeros). To the extent such pulses do occur adjacent to each other, such a signal may be noted as a bipolar violation. Test sets applied to digital transmission cables may detect the number of bipolar violations over a predetermined period of time to test the operational integrity of the transmission lines.
There may be many miles of cable between the central office and the customer premises, with a large number of repeaters between the two facilities. Thus, if the malfunction of a transmission line is detected during a test (or simply during normal operation), it is important to make an accurate determination of the location of the fault. In this way, the fault may be located and corrected more quickly and inexpensively.
Furthermore, to assist in the testing of transmission lines and correction of faults, a technician may wish a repeater, an NIU or another transmission line element to identify not only its location with respect to the test set, but also (or alternatively) its condition. For example, a repeater may be able to enter a particular operating mode, such as xe2x80x9clogical loop backxe2x80x9d or xe2x80x9cmetallic loop back,xe2x80x9d and a technician may command the repeater to communicate back to the technician that it is in such a mode.
Also, for example, if a repeater detects that an adjacent repeater or span of transmission cable is not functioning, the repeater may move to an open power loop mode, signifying that the adjacent span or repeater is malfunctioning. The repeater that is reporting the malfunction should be able to identify itself to a testing technician so that the technician may more readily locate the fault.
Further, a repeater may, for example, be in a loop back mode as result of commands issued by a first technician. A second technician may wish to know which repeater has been placed in the loop back mode. The repeater in the loop back mode should be able to identify its location and condition to the second technician, such that more efficient testing and repair of the transmission lines may be effected.
Still further, for example, a technician positioned remotely from an NIU may wish the NIU to report on the status or condition(s) of the network transmission lines and/or the conditions of the customer premises equipment and customer transmission lines. In response, the NIU should be able to communicate such information to the requesting technician.
Unfortunately, many of the presently available apparatus and methods for communicating with transmission line elements, such as repeaters and NIUs, are cumbersome and expensive to manufacture. The presently available reporting systems often substantially increase the size, weight, and complexity of the line elements. Moreover, such systems may involve the use of specialized codes, such that the technicians must utilize modified line elements as well as new, specially designed test equipment in order to allow the test equipment and line elements to communicate with each other.
In a principal aspect, the present invention is an apparatus for allowing a line element, such as a repeater or NIU, to communicate with other equipment at a remote location. The apparatus is interconnected to a digital transmission line carrying a stream of coded data and preferably includes both a detector and an error generator. The detector senses a status inquiry and responsively produces an initiation signal. The initiation signal is received by the error generator, which, in turn, responsively introduces into the data stream an error or burst of errors. The error or burst of errors corresponds to status information, which may include, for example, a code identifying the particular element, the location of the element, the operating mode that the element happens to be in, or the operating mode of the transmission lines interconnected to the line element. This error or burst of errors may then be detected by remote test equipment.
In another embodiment, the invention is a repeater interconnected to both incoming and outgoing digital transmission lines. Again, the incoming transmission line carries a data stream of coded signals. The repeater includes a build-out circuit as well as the detector and the error generator. The build-out circuit receives the incoming data stream and responsively produces a repeated data stream along the outgoing transmission line. The detector senses the inquiry signal and responsively causes the error generator to introduce an error or burst of errors into the repeated data stream. The error or burst of errors again corresponds to the status of the repeater. Again, the status may be either the location or an identification of the element, or the operating mode or other information regarding the repeater.
More generally, an aspect of the present invention is a method of transmitting status data from a transmission line element to a remote location. The method includes detecting a status inquiry and responsively introducing a predefined set of errors (xe2x80x9cerror messagexe2x80x9d or xe2x80x9cerror response messagexe2x80x9d) into a data stream carried by the digital transmission line. The error response message corresponds to the status of the transmission line element.
Thus, an object of the present invention is an improved remote reporting system for a digital transmission line element. Another object is an improved repeater that is smaller and lighter in weight.
Still another object is a line element such as a repeater or NIU that may communicate with a remote location with less need for modification of the line element and with less need for the use of specialized circuitry to decode the signal generated by the line element. Yet still another object is a line element that may more easily communicate with presently available testing equipment regarding its status or mode of operation. These and other objects, features, and advantages of the present invention are discussed or apparent in the following detailed description.